I'm not so sure al Qaeda is more dangerous simply because it is decentralized.
After all, raising funds without a central paymaster, researching targets without central guidance, planning without the benefit of strategists, obtaining weapons and destructive materiel without a centralized supply system, maintaining morale without charismatic leadership and operating without the general support of al Qaeda, "the Base", as a whole will, in my opinion, make al Qaeda terrorist cells less dangerous.
Of course, that doesn't mean these cells aren't dangerous, nor does it mean they won't kill lots of people if allowed to continue operating. Nonetheless, I was under the impression that the very centralization itself, the charismatic leadership of Usama bin Laden, sophisticated C4I, training and resources that made al Qaeda so dangerous compared to other terrorist organizations.
Now that the head has been severed, the tentacles may wriggle, but they can no longer reach out together.
I agree. Shining Path guerillas of Peru went down the tube after their top leaders (21 of them ?) were all captured by Fujimori gov. We could see many poorly planned ineffective attacks with less damage, which will end up exposing more of their operatives than are required under a careful planning. In the end, the lack of successful attacks will demoralize operatives themselves.
If they are truly demoralized, they could quit terrorism and go into organized crime as a career change. All of their expertises are quite useful in organized crime. The only thing missing is a "cause", which they would have lost by that time.